A Survey of Stone Consolidation Methods: The case study of a sculptural monument at Forest Lawn Cemetery

A Survey of Stone Consolidation Methods: The case study of a sculptural monument at Forest Lawn Cemetery by Lauren Paige Isaacs

All right, so I’m real excited to be sitting here and kicking off materials section. As a conservator, that’s how I think on a regular basis and see the world around me, so I’m ready for materials. Anyway, this is also my graduate research that I did as a student in school at Buffalo, and what’s awesome is I’m a native Nashvillian, so I actually get to come home and present on my graduate topic. It’s perfect.

All right. All right, so a bit of background about it. I went to school at Buffalo State College, which is one of three comprehensive programs for training comp conservators for fine arts in the US. I was trained in paper, paintings, photographs, textiles, and the all encompassing objects, which was my major. Objects is a lot of different materials, which I, sorry, which I wanted to explore fully.

I came from a background mostly in ethnographic working with American Indian and Oceanic works of art, and then more recently, I’ve been working in the modern and contemporary realm, which involves plastics and modern paints and all sorts of mixed media. So, coming from this background and knowing that was kind of what I was doing, and I wanted to try something completely different for my thesis at Buffalo. So, I thought about it, and one thing that was lacking in my education was stone conservation, especially in an outdoor environment. So, this is what I’ve decided to pursue, really with no other background other than a general sense of materials.

All right. So, why stones? Stone to me is what preserves cultural heritage best over time. I think the most authentic accounts we have of a civilization’s records, their religious beliefs, their customs, artistic vision, all of that has been transpired in stone for millennia. Also, it has the ability to make amazing works of art, especially in marble and dolomitic stone, which is where the scope of this research is.

One of the problems though is marble and dolomitic stone is a lot more susceptible to the environment than it was at the time when these works of art were created due to advances in industrialization. There are three main causes of deterioration in stone. Soil and salts, biodeterioration and air pollution, which is what I’m focusing on here. And how do you get sulphoric precipitation? This comes from gases emitted from pollution. It goes up in the air. It combines with water that’s in rain clouds, and hydrolyzers form hydrogen, sulfuric acid and nitric acid, which then comes down and rains on works of art.

Forest Lawn Cemetery at Buffalo, NY

Once it does this, the exact mechanism isn’t really known, but what’s believed to happen is the acid rain hits the surface of the marble or dolomitic stone. It forms a soluble salt, which is then washed away, leaving behind little small sinks of surface area, which causes, unfortunately, more surface area and more chemistry to occur.

Despite the obvious difference in subject matter here, this is also a good example of kind of the difference in what happens when you lose the surface. On the left you have a Hellenistic marble sculpture that, even though it was exposed to outdoors at some point, somebody brought it in, and it never really got exposed to acid rain. On the right, you have a figure which is part of the memorial that I’ll be talking about that was exposed to acid rain, has now what is known as “sugaring”, because it appears to look like sugar, but that’s from losing the small grains over time.

So, this project … I got interested in this project after I saw a monument at Forest Lawn Cemetery, which was in Buffalo. Forest Lawn Cemetery is approximately 269 acres of central Buffalo land, and it has about 153,000 permanent residents. It was created after Buffalo saw a rise in their population following the completion of the Erie Canal in 1825, and today, it’s nationally recognized as a historic landmark. The Pratt Memorial which is what I’ll be speaking about is one of the most elaborate and well known monuments at the cemetery, and it is also visible from another section of major highways. It was erected in the late 19th century by Samuel Fletcher Pratt. He was a steel supply company owner, and it is there to commemorate him and his extended family. The monument contains four central figures, up here at the corners and one central figure in the middle that’s covered by a Gothic architectural element.

The Pratt Memorial

So, this is definitely the best case of what is known as wet and dry deposition, and I just wanted to spend a minute and talk about the differences between the two, because I think that it’s something that isn’t really described in literature, and a lot of conservators don’t know the difference between the two either. As opposed to a wet deposition, which is what I was describing where the gases go into air, they combine with the water, and then you get snow, rain, by the way it’s precipitation because it’s not just rain. Snow and ice actually may have higher concentrations of the gases and are in contact for longer periods of time with the surface, so it actually might be worse, like Buffalo.

Anyway, so that’s wet deposition, and dry deposition actually happens when something’s covered and those soluble salts that are forming on the surface from just moisture that’s in the sculpture, when that reacts with the gases in the environment, it traps in carbon and soot and other particulates in forming what is known as a gypsum crust. The difference is the surface doesn’t get washed away. You still get this really intricate detail, but there’s a difference in material between the gypsum crust and the actual dolomite stone that’s underneath. Because of that, you get large losses, entire hands, huge areas of detail that just fall off over night, so it’s a little different, even though it’s the same principle of acid rain eating away at the surface.

So again, this paper is just on wet deposition which causes gradual erosion as opposed to the usual erosion. So, auxiliary areas surrounding were not covered, and so all of them have lost some surface detail if not all. This is just a close up to show you that sugaring and since it’s been going on so long, these areas are becoming uneven, and just over time that just builds and builds and builds until you’ve lost all integrity of the surface.

For the working analysis for this project, first I had to characterize the stone of what the memorial was made out of. I took samples from all the figures, and they ended up being the same thing through x-ray defractual analysis which was delimited limestone. So, in order to do my test, I needed to also get some delimited limestone. Luckily, with the help of Erik Salvid who will be presenting here, I was given four great samples, one rainy day in Connecticut. Of these samples, although they were all sugaring at the same right, and looked to be in pretty bad shape just like the monument x-ray analysis revealed the differences. Of the samples, one was dolomite, and dolomite has a content … because limestone is a sedentary rock, it replaces the calcium content with magnesium content, and based on the percentages of magnesium in the stone, that’s how you classify it as dolomite, which one was. Calcite which is less than 10% magnesium and dolomite limestone which is 10 t 45% magnesium, so of the four samples, 50% turned out to be just what I needed which was more than limestone could produce for these tests.

When you’re looking for a consolidate, there are things you want an there are things that you would like to have happen. It would be nice if you had other things, but that’s virtually impossible. A consolidate must penetrate through the surface. It must increase security between the loose materials and the substrate beneath. It must also maintain adhesion to the actual substrate that you’re trying to consolidate, and it shouldn’t change or interfere with the mechanical properties of the stone which in an environment can cause disastrous effects. It would be nice if it was also stable and durable in that outdoor environment and if it wouldn’t discolor the white pristine color of the stone. Also, if it’s affordable, available and safe to use, because many are not.

Alkoxysilanes

With the project being chosen, based on literature, reading and knowing the problems of the stone, our alkoxysilanes which is not a new concept but recently has been published about by George Wheeler through Getty Research or Getty Conservation Institute, also an idea that was presented in there was consolidating with alkoxysilane and parallel alkoxysilane with Paraloid B-72, an acrylic polymer epoxy which would lower viscosity that’s applied in a solvent, airmatic alcohol and carbon mix which is what I chose, and lime water, which is basically calcium hydroxide once it’s exposed to carbon dioxide in the atmosphere, it performs the limestone.

So, alkoxysilane’s really quickly. Again, I found this in the Getty publication. A bit of chemistry, they form siloxane bonds with a silica present on the surface of the stone. With limestone, there’s not a lot of silica present as there is in sandstone. There’s no voids or anything, so you have to give it a pre-treatment step in order for it to happen. But the nice thing is once it forms, it happens through the internal moisture of the stone due to account which water. Then you let the bonds … You’re not adding too much. It’s pretty safe. But like I said, the dolomitic stones, you need a pre-treatment step, because there’s not those silica bonds to form. Using ammonia tartriate, which is in the step called HTC hydroxyl finishing, or, yeah … It’s the finishing rinse, hydroxy conversion treatment, sorry, and then once that forms, you get your silica bonds, and then you can apply your alkoxysilane.

Alkoxysilane work by these areas that are open up by acid precipitation rinsing out various ions and salts that are holding these calcite crystals together forming a lot of very small bridges between which overall builds the strength and adds good quality to the stone, because it penetrates pretty deeply. But one problem with alkoxysilane, and especially the problem that I was facing with delimited limestone is the pore sizes are sometimes too large to be bridged by such a small bond. So, it’s not even, and it’s not a complete bond. Again, just to review, the advantages are that they are chemically stable, low viscosity. They can penetrate deeply. They won’t discolor the stone. Disadvantages, they cannot bridge large gaps in more porous stones, and it’s a poor consolidate for carbonate rock, which requires pre-treatment steps, and it takes a little while.

Again, this is why I decided to try a acrylic resin and the alkoxysilane to see if that would bridge the step too. The advantages of using this, B-seventy-two, if you’re a conservator, a novice conservator, it should guarantee you from pretty much everything. And because of that reason, that’s why I wanted to try it out.

Epoxy

It solutes to a wide range of solvents. It’s clear. It won’t discolor when it’s exposed to UV or oxidation. It has both good adhesion, cohesion. It’s been shown to last up to twenty years in a natural environment, yet still be reversible. Disadvantage, it’s a large molecule. It’s a polymer, so it won’t go down as deeply as alkoxysilane alone. It can cause a surface change since it is basically a plastic. It has a low blast transition temperature of 104 Fahrenheit. So, I would not recommend it in environments like the South, where it gets warm in the summers, and also if it does get to that point, it’ll attract dirt, dust, other particulates. Also, one of the things that’s being explored more and more is long term exposure to water. Since it’s plastic, it’ll take on water, expand, contract, but it might also cause additional spalling.

The other option, epoxy, a lot of people think epoxies aren’t necessarily the best thing for a consolidant, but there’s actually some advantages to it. The one I tested here is Epotec 301 from Epoxy Technology. It’s another go to for objects conservators. Advantages, it’s very durable. It has good adhesion, cohesion. It improves the mechanical strength of the stone. It has … it’s been shown to have preventative aspects to it in blocking out those bad gases like sulfuric oxide. Disadvantages, it can be difficult to apply or scale for a monument. Epoxy solvents in application is conservative to a much larger extent than other things, and residual epoxy left on the surface won’t yellow or change any of its physical properties. It’s not easily reversible, and if you did have to reverse it, sometimes it’s not the safest of solvents.

Going with epoxy, there are two different ways that people describe using them. One was in America, a lot of the thinking was if you use a faster evaporating solvent it will cause the polymer to stay in the stone as the solvent evaporates. The other thinking was from Europe, using a slower evaporating solvent, high in hydro carbons and wrapping it in polyethylyne sheeting, causes the vapor pressure to kind of push the molecules and eventually keep them in there. Based on the results I read, I went with the European model. It just seems like it ha better results in the end. And finally lime water, which like I said is calcium hydroxide. Also, I did the decycling test that is in an environment such as Buffalo to see what it would be like with pre-fall cycles, and there was no UV involved, just the …

Similar results, physical examination, controlled salt, no visible surface change. alkoxysilane is all pretty much the same as the control. The acrylic resin didn’t pull as much, but did kind of have that plastic sheen that I mentioned. The epoxy, no real difference, but again, there was no UV exposure, so I can’t really draw any conclusions from that. Lime water was so hard to do that I couldn’t do it through the pre fall cycling, but as you can see it does have that spongy network.

Abrasion Testing

Another final test was low viscosity epoxy with the controlled, there was little difference. Epoxy cylene coated the surface. This jelling, this cracking has been noted. NO real long term effects are known from it. With the acrylic resin, the acrylic resin didn’t show up on SEM, but also good coating, good penetration. Low viscosity epoxy coated really well. It penetrated really deeply. These are all samples that I took from about half an inch into the stone. Final test I did was spot abrasion. I did this using a modified ASTM standard. After doing this, the loss in volume based on a certain distance and pressure, the larger the bar the more loss, so the weaker the treatment was. Of this, you’ve got low viscosity epoxy and acrylic resin and alkoxysilane that worked out pretty well.

So, the winner in my opinion is alkoxysilane, and I think this is based n a lot of factors, mainly the environment in Buffalo and I know how B72 will work in that environment as opposed to other environments. I wouldn’t recommend it for everyone, but based on this case, it worked out the best. So, my final thoughts are that the goal of this project wasn’t to discover a breakthrough in stone conservation.

There are so many sources out there. There is so much information. It is time consuming and tedious to go through each and everything, and in the end, the literature only gets you so far. There’s no one source. All I can advise you on is to know the basics. You have to know your stone. You have to know what options are out there, and don’t rely on proprietary promises as every place is different, and I hope that doesn’t get you in trouble. It’s risky, and we don’t really know what’s going to happen in the end. I mean, as with most consolidation treatments, reversibility is not a possibility, so what you are putting in there will have some form of long term effect. Epoxies can discolor and B72 can attract dirt. However with this case, I knew going into it, you can’t just let the surface erode away either. So, anyway, that was my research. These people I would like to thank. Thank you.

Abstract:
As a conservation student working with a generalized knowledge of outdoor stone deterioration and viable treatments options, Ms. Isaacs confronts the problems of acidic deposition on calcareous stone using the Pratt Memorial at Forest Lawn Cemetery as a case study in Buffalo, New York. The selected monument was erected in the late nineteenth century and has since suffered from prolonged exposure to acid precipitation resulting in severe granular disintegration (also referred to as ‘sugaring’).

For this project, the original stone was classified as dolomitic limestone using XRD (X-Ray Diffraction) analysis. Comparable weathered stone samples were obtained and similarly classified with XRD for evaluating potential treatment options. After a review of current and traditional treatment approaches for granular disintegration of limestone, four possible consolidation treatments were selected for evaluation. These options included: lime water washes, low viscosity epoxy in solvent mixture, commercially available alkoxysilanes, and alkoxysilanes with the inclusion of acrylic resin (Paraloid B-72).

Following treatment, the samples were subjected to accelerated aging using a series of controlled freeze thaw cycles to approximate a climate, such as Buffalo, over a twenty year period. The samples were then evaluated against each other and a negative control for changes in surface appearance by using photomicrography, depth penetration and overall coverage by viewing cross-sections with SEM (Scanning Electron Microscopy), and imparted strength by using a modified ASTM International Testing standard for spot abrasion.

The results of this research indicated that for this particular case, alkoxysilanes with the inclusion of acrylic resin performed the best overall. From this investigation it is evident that stone, its deterioration, and possible treatment options cannot be generalized. For every case of stone degradation in an outdoor environment it is crucial not only to accurately characterize the stone in question, but also the environment it is exposed to over time, as well as consider how the two will interact with the inclusion of a consolidation material.

Author Biography:
Lauren “Paige” Isaacs specialized in the conservation of objects at Buffalo State College where she earned a MA and CAS in Art Conservation. She grew up in Nashville, TN and received a BA from Washington University in St. Louis in 2004. Paige was compelled to pursue a career in Art Conservation after studying abroad in Siena, Italy followed by a pre-program summer internship at The Frick Collection in New York City. She spent the following year as a conservation intern working full time at the Denver Art Museum in Colorado before entering a program of formal training.
While pursuing her master’s degree, Paige was a part of the Gulf Coast Recovery Project, where she assessed, stabilized, and treated museum objects for various institutions across the Gulf Coast affected by Hurricane Katrina. After completing her formal training in Buffalo she moved to New York City to complete a practical year of training while working at the Museum of Modern Art. Paige continues to live in New York City working as a freelance conservator through her private practice for various museums, galleries, and private clients throughout the area and beyond.